Image representation in primary visual cortex depends critically on the spatiotemporal pattern of convergence of lateral geniculate axons as well as on the dynamic properties of thalamocortical synapses. Thalamic input represents only a fraction of the excitatory drive to cortical cells in layer 4, but dominates their visual response patterns. We will use electrophysiological methods in vivo to record visual responses simultaneously from thalamorecipient neurons in primary visual cortex and several of their input cells in the lateral geniculate nucleus. In Aim 1, we will test the hypothesis that simple cells in layer 4 have a larger spatiotemporal footprint of input inhibition than excitation. In Aim 2, we wil characterize the properties of the postsynaptic potentials from single LGN neurons onto excitatory and inhibitory simple cells in layer 4. We will also study their dynamic properties such as short term plasticity and synaptic integration from multiple LGN cells. Finally, in Aim 3, we wil use dynamic clamp in layer 4 simple cells to study convergent input from combinations of single LGN neurons (using knowledge obtained in Aims 1 and 2) without concomitant activation of cortical circuits. We will estimate the relative contributions of synchronous LGN input and local inhibition in establishing L4 simple cell visual responses. These studies will generate critical insight into the transformation of visual information that takes place in the thalamocortical synapse.